Temperature control system



Sept. 30, 1958 c. H. BAINES 2,854,558

TEMPERATURE CONTROL SYSTEM Original Filed Sept. 14, 1953 2 Sheets-Sheet 1 CHA PL ES H. BA/NES:

1N VEN TOR.

C. H. BAINES TEMPERATURE CONTROL SYSTEM Original Filed Sept. 14, 1953 Sept 30, 1958 2 Sheets-Sheet 2 w R M M my MW 0 m m m V- B C C W, r

A T TORNEVJ' United States Patent TEMPERATURE coNTRoL SYSTEM Charles H. Baines, Whittier, Caiif., assignor to Harlio Manufacturing Company, Venice, Calif., a copartnership Original application deptember 14, 1953, Seriai No. 380,065, now Patent No. 2,743,933, dated May 1, 195d. Divided and this appiication February 27, 1956, Serial No. 568,035'

6 Claims. (Cl. 219-) The present invention relates to an improved temperature control system for producing an indication or a control efiect Whenever a selected temperature condition is not met.

Thisapplication is a division of copending application Serial No. 380,065, filed September 14, 1953, and entitled Automatic Temperature and Pressure Control Circuit for Sheet Metal Forming Tools.

One practical application of the temperature control systemof the present invention is in a metal forming tool, and the system may be used to render such a tool inoperative whenever certain temperature conditions are not met. As an illustrative example of the control system of the present invention, one embodiment will be described as employed in a dimpling tool for conditioning sheet metal to receive flush rivets. It is to be understood, however, that the system is not limited to that type of tool but will find other applications.

In certain examples, it is desirable that any rivets used therein be flush with'the surface of the sheet metal through which the rivet is passed. This is particularly true in the aircraft industry where streamlining is of paramount importance. In order that the rivets be flush with the metallic surface, a conical depression is first made in the metal for housing the rivet head and this is known as dimpling. This operation permits the rivet to be properly driven into place with its fiat head flush with the sheet metal surface. In some types of sheet metal, such as titanium alloys used in aircraft construction, dimpling can cause considerable damage to the metallic sheet unless it is preheated to a temperature where it is sufiiciently malleable to conform to the dimpling die without fracture.

One principle difliculty with prior art dimpling tools is that the preheating control system does not contain any reliable provision for preventing operation of the tool in the event of failure of the preheating system. If the tool is operated over incorrect temperatures or when the metal is cold, the dimple cannot properly be formed and the whole sheet of stock metal may be damaged and wasted. This is not only costly due to the high price of such metals but is also time wasting and ineificient.

An object of the present invention is to provide an improved temperature control circuit that may be used, for example, in the tool described above to prevent operation' of the tool when the correct temperature conditions are not met.

A more general object of the invention is to provide an improved temperature control system for producing an indication or a control effect whenever a certain temperature condition falls below a selected level.

These and further objects and advantages of the invention will become clear upon referring to the following detailed description and to the accompanying drawings, in which:

Figure 1 schematically illustrates the dimpling tool in the dwell time phase of operation;

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Figure 2 shows the position of the tool at the time the dimple is formed;

Figure 3 is a schematic circuit diagram showing the control circuits for effecting the dimpling operation;

Figure 4 shows an alternative type of electronic control circuit which may be substituted for one of the control circuits of Figure 3; and

Figure 5 shows a front view of the panel of a suitable frame for housing most of the circuitry for the tool.

In Figure 1 there is shown a sheet of metal 10 in which it is desired to make a dimple for housing the head of a rivet to provide a flush surface. The dimpling tool itself may be made portable so that an operator may move it about relatively freely to spaced points along the sheet 10 for rapid dimpling thereof in succession. As shown, the tool comprises a male die member 11 having a conical projection 12 terminating in a pilot 13 adapted to pass through the rivet marking hole in the sheet 10. 0n the opposite side of the sheet 10 there is provided a mating female die 14 having a mating conical recess 15 and bore 16 for receiving the projection 12 and pilot 13 of the member 11 respectively. The jaws or die members 11 and 14 are thus maintained in proper alignment.

In a preferred construction, the female die member 14 is provided with a heating coil 17 whereby heat may be conducted up through the member to the under surface of the sheet metal 10 to heat this metal uniformly about the rivet opening as at the contacting surfaces 18. Power to the heating coil is provided through leads 17'. Also included adjacent the heater coils is a thermocouple element TH adapted to provide an electrical signal in response to the temperature of the heating coils.

In Figure 2, the first member 11 is shown in mating engagement with the second member 14, the sheet metal 10 having been pressed between the members to form the dimple. This operation may be effected by a piston 19 operating in a cylinder 20 supplied with compressed air through a conduit 35. A tension spring 21 is provided for returning the member 11 to its unengaged position when the compressed air pressure is relieved. The showing in Figures 1 and 2 is purely schematic and the actual construction and arrangement of the piston and cylinder in the pneumatic system may be of any suitable design.

The components of the tool described thus far may be conveniently arranged in a compact unit, the electrical heating coil lines 17, the thermocouple leads (not shown), and the conduit 35 being housed in a suitable flexible sheath and leading to the principal control circuits.

in the dimpling operation, an operating switch on the tool is depressed to actuate suitable control circuits to supply air under a given pressure to the cylinder 20. This pressure will move the member 11 into engagement with the sheet metal 10 and although this given pressure is not sufficient to deform or dimple the sheet member, it is sufiicient to bring about pressural engagement of the sheet 10 with the lower member 14 in order that proper preheating of the metal may be had. The members 11 and 14 remain in this dwell position for a certain dwell time as determined by a timing motor in the control circuits. After the metal 10 is properly preheated, the control circuits are automatically actuated to supply a much higher compressed air pressure to the cylinder 20 and effect the dimpling. The members 11 and 14 are caused to remain in the positions shown in Figure 2 momentarily and then the air pressure is released and the spring 21 returns the member 11 to the unengaged position as hereinbefore explained. The tool is then in condition for making another dimple.

A preferred type of control circuit for effecting the above outlined operations is shown in detail in Figure 3.

In this drawing, the circuit is shown as divided into two main sections by a transformer, the upper section being designated generally by the letter X and the lower section by the letter Y. This arrangement is purely for simplifying the description. In general, the upper section X includes the control circuits for applying the compressed air pressure to the tool, while the lower section Y includes the relay coil controls and other circuits for preventing operation of the tool in the event of failure of certain parts.

For further convenience in the description of this diagram, certain components of the tool are shown separated from the tool. Also, relay contacts are shown separated from their actuating relay coils. The relay coils are designated by the letters a, b, c, and d while the respective contacts which they actuate are numbered with a lettered prefix to identify the actuating relay coil, such as 11-1, 12-2, etc.

Referring now to Figure 3, a left and right hand power line 22 and 23 are connected to a source of electric power such as a conventional 110 volt A.-C. supply as shown at the top of the drawing. Left line 22 includes a main power switch S1 and a fuse F, and continues downwardly terminating at one terminal of the primary of transformer 38. The right line 23 connects directly to the other terminal of the primary of transformer 3%.

Bridging the main line conductors 22 and 23 just after the fuse F is an interrupter motor M the purpose for which will become clear as the description proceeds. Also bridging the main power lines is the heating coil 17 which is located in the tool as shown in Figures 1 and 2. This coil is connected to the lines 22 and 23 by conductors 17 and includes in series therewith relay contacts [7-1.

The compressed air control system for operating the tool is electrically actuated through a lead 24 from the left power line 22, switch S2 located on the tool, relay contacts a-l, and a switch S-3 arranged to be actuated by a timing motor T-l disposed in a shunting line 25, the lead 25 continuing over to the right power line 23 to which it is connected. Lead 24 also includes a series connected second timing motor T-Z arranged to actuate a second switch 8-4. The lead 24 then connects with lead 25 as shown. Branch conductors 26 and 27 are connected between each side of the switch 8-3 and solenoid operated valves Sol1 and Sol-2 respectively in the compressed air control conduits.

The compressed air system itself comprises a source of compressed air 28 having an outlet conduit 29 passing into the solenoid operated valve Sol-1. A branch conduit 30 forks off from conduit 29 before Sol-1, passes through a pressure regulating valve .31, pressure gauge 32, and a check valve 33 to the second solenoid valve Sol-2. The outlet sides of the valves S014 and 801-2 are connected by a conduit 34. Approximately at the midpoint of the conduit 34 there is provided a T-joint from which the conduit 35 passes to the tool as shown. The solenoid valve Sol-2 is a three way valve, its third opening comprising an exhaust vent 36. A return electrical lead 37 for the two solenoid valves passes up through the series connected switch 8-4 to the lead 25.

In the X section of the circuit of Figure 3 there is provided a pilot light P-l shunting the primary of transformer 38 for indicating when the switch Sll is closed.

Referring now to the Y section of the control circuit, the secondary of the transformer 38 connects to left and right power lines 39 and 40. The line 40 includes a series connected current limiting resistance R and thence branches to connect to rectifiers D1 and D-2. These rectifiers provide D.-C. current for operating a pyrometer circuit and the various relay coils. As shown, the rectifier D1 is connected into a line 41 including in series progressing from right to left, a voltage dropping resistance R4. relay actuating coil a, shunted by a small condenser Co-1, and thence through relay contacts 01-1 to the left power line 39. Following down the left power line 39, there is a branch lead 42 connected to two pyrometer coils shown schematically as 43 and 44 in a pyrometer PY. A main temperature indicating needle N is carried by coils 43 and 44 but is shown separated therefrom to clarify the drawing. The coil 43 is shunted by the thermocouple element Th. The other end of coil 43 is connected to a lead 45 including a fixed resistance R-ll and a variable resistance R-ll2 in series. From the resistance R-12, line 45 connects to right power line 40 through relay contacts d-2.

The end of the other pyrometer coil 44 connects to a lead 46, high limit pyrometer contacts HL, interrupter switch 47, relay actuating coil 12, and voltage dropping resistance R4. to the right power line 30. Relay coil 11 is shunted by a small condenser Co-Z. As shown just to the right of the pyrometer PY, there is provided a con ductor 48 including a second interrupter switch 49 and low limit pyrometer contacts LL in series all connected between the conductors 41 and 46.

A branch lead 50 from the right side of resistance R4 in lead 41 includes a condenser Co-3 and connects to a line 51. Line 51 includes series connected condenser (10-4 and connects at one end to the right power line 40 on the negative side of the rectifier D2. The other end of line 51 connects to left power line 39 adjacent the secondary of transformer 38 through branch lead 52. A condenser Co-5 is connected between line '51 and the conductor 48 between the interrupter switch 4-9 and low limit LL contacts of the pyrometer effectively shunting the low limit LL contacts and pyrometer coil. The pyrometer coil 44 and high limit contacts HL are shunted by a condenser Co6 connected in lead 53 as shown. These condensers prevent sparking at the L-L and HL contacts.

Continuing on down the left power line 39. a lead 54 passes through relay contacts d-3, heating coil 11 in a thermal contact control e, and a pilot light P7. to the right power line 40. The pilot light P2 is shunted by relay contacts a-Z connected in shunting lead 55. To the left of the heating coil 11, a branch lead 56 includes relay contacts 0-1 and is connected between the line 54 and the conductor 46.

As shown in the lower left hand corner of the Y section, the relay actuating coil 0 is connected at one end through a voltage dropping resistance R3 to the left power line 39 as by conductor 57. The other end of coil 0 passes into a branch conductor 58, relay contacts [2-2. and thermal actuated contacts e-1 to the right power line 40. This other end of coil c is also connected to lead 59 which includes in series, relay contacts c2 and relay contacts a-3, and thence connects to right power line 40. The thermal contacts e-l and relay contacts a-3 are shunted by a pilot light P-3 connected in conductor 60, this latter conductor connecting to leads 58 and 59 through lead 61, and to the right power line'40.

Referring again to the lower left hand corner of the Y section, the relay actuating coil d is connected at one end through voltage dropping resistance R-4 to the left power line 39 as by conductor 62. The other end of coil d connects to the right power line 49 through relay contacts d-4 and through a conductor 63. A small rectifier D-3 is provided in a line 64 connected between the conductors 59 and 63 adjacent the ends of the relay coils c and d.

The various relay contacts are shown in the positions they assume when their operating relay coils are deenergized.

Operation In the operation of the dimpling apparatus and the control circuit therefor, the main power switch 84 shown in the upper left hand corner of Figure 3 is closed starting the interrupter motor M and lighting the pilot light P-1 to indicate that the power is on. Heater 17 is also portion of the Y J energized through the normally closed relay "contacts b1. This heater-willserve-to heatthe metallic sheet 10 shown in Figure 1 to a propertemperature to place the metal in condition for dimpling. The compressed air system is not energized because of the normally open switch 8-2 on the tool, which switch is manually controlled by the operator of the tool.

Closing ofthe main'power'switch S-I'Will energize the primary of transformer 38 and place power on the secondary lines 39 and 40. Relay coil a in lead '41 will accordingly be energized through'the normally closed relay contacts d-l, voltage dropping resistance R-1, and rectifier D- l. Energization of relay coil a opens the contacts a-l-in lead 24'of'section X of the circuit to insurethat the-tool cannot be-operated even though switch 8-2 may be inadvertently closed by the operator.

The purpose of disconnectingthe tool from the circuit inthismanner is to give the heating coil 17 a chance to come up to'proper temperature and thus prevent dimpling of the metal in a'cold='state. The fact that'contacts d-1 are closed will insure energization of relay a. T hesecontacts shunt the pyrometer coil 44 and L-L contacts whereby the needle "N is free'tom'ove since the voltage across coil 44 is shunted-out. The L-L contacts being normally closed at the start.

'When relay coil a is energized, the normally'open relay contacts'a-2 at the middle right'hand portion of section Y of the ci'rcuit'are closed shunting out 'pilot light P-2 indicating that the tool is not ready to be operated. The contacts a-B at the lower right hand portion of the Y ci'rcuitareopened.

After heating coil 17 has attained the proper temperature,the current"from' thermoccuple TH shunting the pyr'ometercoil'43 in the Y section of'the circuit causes "thenee'dleN towing and close the high limit H-L contacts in lead -46. Thisthermocouple,-as shown in Figures 1 and Z, is disposed adjacent the'heating coill7 in the tool and is adapted to close the"H--L'contactsthrough current supplied'to the pyrometerc'o il 43 swing needle Nonly when a certain predetermined temperature has been reached.

Closing of the 'H-L contacts applies voltage across pyrometercoil- 44 holding theHL contacts closed and energizes relay-coil b in -line- '46 through the pyrometer coil 44, H- L contacts, interrupter'switch '47, and-voltage dropping resistanceR-Z. This relay coil then actuates 'the'relay contacts b-l to "openthe' line 17 containing the heating coil 17 thereby removing the coil from the circuit and preventing over-heating. Atthe' samc time' relay contacts b- 2-at thecenterlowerportion of-the-Y section are 5 closed by'relay b to'energi'ze relay coil c through conductor 57, voltagedropping resista'ncdR-3, conductor 58,

"and thermalcontacts e-l which are normally closed.

Energization of relay coil c closes its relay contacts -2 to lock itself in energized condition through lead 53, lead 61, and the normally closed thermal contacts e1, the

'relay' contacts n 3" being now in open condition.

Relay coil d'is also energized upon the 'closingof relay 7 contacts b-2,through con'ductor62, voltage dropping resistance R-4, line 64,rectifier D3, line 58, contacts 11-2, and thermal contacts e 1. The rectifier D3 acts as a check valve preventing relay coil cr'rorn being energized through lead 6.3 when relay contacts a-3 and thermal-contacts -e1 are open, the relay contacts d-4 in 63 being closed by energization of coil 01 to lock itself in throughconductor 63. This relay d- -also opens relay 'contacts d '2 in line 45 removing voltage from the pyrorneter- PY coil'43. Relay contacts d-l are simultaneously opened to de energize relay coila in lead 41 at the upper section of the circuit. Dc-encrgization ofrelay coil 0 closes relay contacts a-lplacing the tool in condition for operation upon'manual closing of the "switch'S-2as shown in the X section of thecircuit. "energization of this relay'a also closes relay contacts 11-3 s and opens relay contacts a'2-so that these contactsassume the positions as shown in solid line in'Figure 3.

At this time it is to be noted that the relay coil d et the bottom of the Y section of the circuit closes the relay contacts d-3 in line 54 to light the pilot light P-2 through the heater h in thermal control e,'the relay contacts a-Z now being open. This pilot light P2 visually signals the operator that the heat is proper in tool heating coil 17 and that the tool may be operated at any time by depressing the manual switch S2 on the tool.

The circuit described thus far provides for a proper heating of the coil '17 on the tool before the tool can be operated.

When it is desired to make a dimple, the operator simply depresses the switch S-2. Closing of this switch starts the timing motor T1 through'conductors 24 and 25. This timing motor is arranged to close switch 8-3 after a given period of time. Closing of the manual switch S2 also energizes the solenoid -:operated valve Sol-2 through conductors 24, 27, return lead 37, and normally closed switch S-4. 'Energization of S01 2 places the conduit 30 in-communication with the conduit 34 andconduit 35 to the tool permitting compressed air from-the air supply 28 to pass through conduits 29, 30, 34 and 35 to the tool. The pressure of this air is accurately regulated by the regulator 31 and indicated by the gauge 32 in the line 30. Sol-1 is normally closed whereby the air must follow the above described paths.

The regulated pressure is not so great .as to cause a bending of the sheet metal by the tool, but just suflicient to insure a good contact between the member 14 and the sheet metal 10 as at 18 as shown in Figure l. The heating coil 17 thus heats the metal 10 by conduction through themember 14 and contacting surfaces 18. The length r of time for heating, referred to'hereinas'the dwell time, is determined by the timing motor T-l.

After this given dwell time has elapsed, the timing motor T1' closes the switch -S3 as shown in Figure 3 to cause energization of the solenoid operated valve Sol-1 through conductor 24, switch v8-3, branch conductor 26, and return lead 37 to the right power line-23 through the sWitch'S-4. S014 is thus opened to bypass the regulator 31 and pass a much greater air pressure fromsource 28. This air pressure is applied to the tool through line 35 and is prevented from backing .up through the still open Sol-2 valve and line 30 by the check valve 33. The greatly increased air pressure serves to eifect the dimpling operation.

It will be noted that upon the closing of switch 5-3 to energize Sol-1, the timing motor T-2 is simultaneously connected across the power lines by conductors 24 and 25. This timing motor T-2 has a relatively short period of from anywhere from one sixtieth to three seconds and times the period that the die members 11 and 14 remain in the position shown in Figure 2. It is important that the increased-dimpling pressure beapplied and maintained for a given period to insure that the dimple will set.

After this given period, timing mot0r T-2 will open switch 8-4 to break the return lead 37 from Sol-1 and Sol2 de-energizing these valves. Sol-1 thus closes and Sol-2 disconnects conduit 30 from 34 and connects 34 to the exhaust vent 36. The'tension spring 21 in Figures l and 2 may then return the member 11 to its separated position, the compressed air in cylinder 20 backtracking through conduits 35 and 34 out the exhaust vent 36.

When the operator releases the manual switch 8-2, the tool circuit is opened and the timing motors T l and T2 are caused to return to their inoperative conditions readyior another cycle. 8-3v is thus opened and-S-4 is closed. The tool is now ready to be moved to a new location and efiect another dimple.

Referring to Figure 4 there is shown an alternative type of timing circuit which may be substituted for the compressed air' control circuit-enclosed inthe dashed line and labeled TC in Figure 3. In Figure 4 the same reference numerals are used to designate the corresponding elements shown in Figure 3. As shown, the leads 24, 26, 27, 37, and 25 are the same as well as the timing motor T-l and switch S3. Instead of the timing motor T2 which determines the time the die members of the dimpling apparatus are engaging the metal sheet to form the dimple, there is provided a thyratron electronic timer which serves to operate a relay contact F-l in line 37 which relay contact replaces the switch S4 of the TC circuit in Figure 3.

This actuation is accomplished by means of a relay coil g connected between the conductors 24 and 37-64 which is energized by closing of the switch S3 upon termination of the dwell time. Energization of relay coil g closes relay contacts g-7 to place the condenser Co7 across a source of voltage delivered to the terminals A and B. Charging of condenser Co-7 takes place through resistance R- and variable resistance R9, resistance R7, and relay contacts g-7. The terminals A and B are connected to the corresponding terminals A and B shown in Figure 3 across the rectifier D-l in the upper right hand portion of section Y. The voltage between A and B is also employed for supplying plate voltage to a thyratron tube V through the medium of relay coil 1 and conductor 68.

The start electrode of the thyratron tube V is connected to one side of the condenser Co-7 through a resistance R-6, the other side of Co-7 connecting to the cathode of tube V. A high plate resistance R-8 is connected between the cathode and plate of the tube as shown.

With this arrangement, plate current will not flow through the relay coil 1 and lead 68 until the tube V fires. Firing of this tube will not take place until the start" voltage appearing across condenser Co-7 attains a certain value. The time at which the tube V will fire is thus determined by the time-constant of the condenser Co-7 and resistors R-10 and R-9, the latter of which is variable to permit adjustment of this time constant. Accordingly, only a predetermined time after the closing of relay contacts g-7 will the tube fire. Upon discharge of tube V, current through the plate circuit of the tube will energize relay coil f to open the relay contacts f-l between conductors 37 and 64 thus opening the return lead from the valve solenoids to release the compressed air pressure as described in Figure 3. The time constant of the circuit may be adjusted to any value, for example, between one sixtieth and three seconds.

It is thus seen that the timing motor T-2 of Figure 3 finds its equivalent in the time constant circuit comprising condenser Co-7 and resistances R-9 and R-10 of Figure 4, while the switch 8-4 of Figure 3 finds its equivalent in the relay operated contacts f-1 of Figure 4.

Thus far the described system has related to the automatic controlling of the proper heating, dwell time, dimpling time, and actual dimpling. The remaining components of the circuit in combination with certain elements already discussed are provided for accurately regulating the temperature of the heating coil 17 to insure that it remains within close temperature limits, and for rendering the tool and heater inoperative in the event of a breakdown.

Temperature regulation The temperature control and regulation is achieved in part by means of the pyrometer coils 43 and 44 and the shunting thermocouple TH in the Y section of the circuit of Figure 3. The voltage across pyrometer coil 43 applied through leads 42, 45, resistance R-ll and variable resistance R-12 tends to up-scale the pyrometer slightly ahead of the actual temperature. Therefore, the heater coil 17 current will be shut off by closing of the HL contacts to energize relay [7 and open contacts b-1, just before the heater has reached its maximum temperature, giving the heating coil 17 a chance to stabilize before current is again applied to it. This voltage across the pyrometer coil for up-scaling the pyrometer needle N may be controlled by the variable resistance R-l2 and thus the time the current is shut off from the coil 17 may be accurately adjusted.

The given maximum temperature to which the heating coil 17 will be raised is accordingly controlled by the pyrometer high limit contacts H-L and thermocouple TH. It will be recalled that the dwell period was initiated by de-energization of relay coil a by opening of relay contacts d-1 in line 41, the de-energization of relay coil a permitting relay contacts a1 to close and connect the tool into the circuit. At this time the relay contacts d2 were also opened removing the above mentioned up-scaling voltage from across the pyrometer coil 43. This removal of the voltage across pyrometer coil 43 is an important feature of the invention in that it permits normal calibration to be restored to the pyrometer.

The interrupter motor M includes a series of cams which periodically open the interrupter switches 47 and 49 to permit the pyrometer contacts to be periodically opened. The duration of opening the normally closed interrupter switches however is just sufiicient to permit the pyrometer contacts to open but not so long as to exceed the time-constants of the condensers Co-l and CmZ shunting the relay coils a and b, respectively, thereby allowing the relay contacts a-l and b-1 to be energized or opened. In other words, the momentary opening of the interrupter switches will not affect the relay coils a and b, in view of their shunt condensers. Opening of the HL contacts periodically by the interrupter motor unlocks the contacts by momentarily removing the line voltage from the pyrometer coil permitting the needle N to fall ofi and close the LL contacts in the event the thermocouple or heater break.

Thus, if the temperature of the heater coil 17 begins to fall below the prescribed amount due to the opening of relay contacts b-1 in response to the closing of the high limit contacts HL, opening of the interrupter switch 47 will open these high limit contacts, which contacts will not again close since the heater coil and thermocouple are below the high limit setting. Thus, after the charge on Co-2 has dissipated through relay coil b, this relay coil will be de-energized permitting the relay contacts 11-1 to close and restore heater current to the heater coil 17. When the heater has almost reached its given temperature again, the thermocouple will close the high limit contacts HL, again energizing relay coils b to open contacts b-1 and remove the heating current to coil 17. The cycle thus repeats and the temperature of the coil 17 is carefully maintained.

Breakdown protection The element most likely to break down is the controlling thermocouple TH. This element, as already pointed out, is normally located in the tool and as the tool is moved from place to place, it is not infrequent that the thermocouple leads as well as the thermocouple element itself become inoperative.

In order to prevent the dimpling operation in the event the thermocouple TH breaks, or in the event the heater coil 17 fails to recover, the pyrometer moving element falls back to the low-limit contacts LL to close these contacts. Closing of contacts LL energizes relay coil a through conductor 42, pyrometer coil 44, conductor 48, contacts LL, interrupter switch 49, conductor 41, voltage dropping resistance R-1, and rectifier D-1, it being remembered that relay contacts d1 are opened. Energization of relay coil a opens relay contacts a-1 to disconnect the tool from the circuit. Also relay contacts 11-2 are closed to shunt out the pilot light P-2, the turning off of this light indicating that the tool is not in operaassesses 9 tive condition. Relay contacts a-3 are simultaneously opened.

It is also to be noted that closing of relay contacts a-2 permits energization of the thermal heater h to commence. This heater has a suificient time lag as not to affect the thermal contacts (1-1 until the heater coil 17 has recovered. However, in the event that the thermocouple THis broken, then thermal heater h will eventually cause the thermal contacts (2-1 to open. Opening of thermal contacts e-1 thus de-energizes relay coil permitting the closing of relay contacts 0-1 in the conductor 56 adjacent the thermal control 2. Closing of contacts c-l by-passes the pyrometer coil 44 and high limit contacts H-L through conductors 54, 56, 46, coil b, and resistance R-2, the relay contacts d-3 being still closed. Current through relay coil b locks this relay in energized condition to maintain relay contacts b-1 open and insure that the heater coil 17 is not energized. Simultaneously with the opening of the thermal contacts (2-1, the pilot light P-3 is energized indicating to the operator that trouble is present. At this point, normal operation can only be restored by turning off the main power switch S-1 and replacing the defective heater coil 17 of thermocouple. Closing again of 8-1 will then place the circuit in condition for normal operation.

It should be noted that the low limit contacts are closed when the tool is started. Should the thermocouple TH be broken at this time, the upscaling voltage delivered through contacts d-2 which are closed at the start, will upscale" the pyrometer. However, removal of this voltage by opening d-2 immediately permits the needle to fall back to close the L-L contacts and start the train of sequences outlined above.

It is thus seen that this described circuit will prevent operation of the tool unless the heater coil 17 is at proper temperature and unless the thermocouple TH is in operative condition.

Figure discloses a preferred arrangement of the control circuit panel, the various pilot lights, main power switch S1, pyrometer indicator, pressure regulator, and pressure gauge, and time setting means for the timing motors T-1 .and T-2, being shown and designated by the same symbols identifying these elements in the other drawings.

Various control circuit variations within the spirit of the present invention will occur to those skilled in the art. The invention is therefore not to be thought of as limited to the particular embodiment disclosed.

I claim:

1. A temperature control system for controlling the current through an electrical heating element including in combination: a temperature sensitive element having a high-limit contact adapted to be closed when the indicated temperature of the heating element reaches a selected value and having a low-limit contact adapted to be closed when the indicated temperature of the heating element drops to zero; first relay means energized by closure of said high-limit contact to interrupt the current flow through the electrical heating element; a time-lag relay energized by subsequent closure of said low-limit contact; and a third relay normally energized by said timelag relay to be de-energized thereby after closure of said low-limit contact for a selected time interval, said third relay closing a contact upon de-energization thereof to energize said first relay means along a path independent of said high-limit contact to maintain interrupted the current flow through the electrical heating element.

2. A temperature control system for controlling the current through an electrical heating element including in combination: a pyrometer having a high-limit contact and a low-limit contact, said pyrometer also having a first coil and a second coil, said coils each causing said lowlimit contact to be closed when the current therein drops to a selected minimum and said high-limit contact to be 10 closed when t-hecurrent therein reaches a selected maximum, a thermo-couple. adapted to be positioned adjacent the electrical heating element and connected to said first coil, and said high-limit contact being connected in series with said second coil; a first relay means energized by said high-limit contact to interrupt the current flow through the electrical heating element upon closure of said high-limit contact, with said second coil of said pyrometer holding said high-limit contact closed; means for periodically interrupting the current flow through said second coil of said pyrometer to eifect a closure of said low-limit contact should the indicated temperature of the heating element drop below a selected threshold; and second relay means controlled by such closure of said low-limit contact for energizing said first relay means along a path independent of said pyrometer to maintain the current flow through the heating element interrupted.

3. The system defined in claim 2 which includes a thermal relay, and in which said second relay means is controlled by said low-limit contact through said thermal relay to energize said first relay means only after a selected time interval.

4. A temperature control system for controlling the current through an electrical heating element including in combination: a temperature sensitive element having a first contact adapted to be actuated when the indicated temperature of the heating element reaches a first selected value and having a second contact adapted to be actuated when the indicated temperature of the heating element drops to a second selected value; first relay means controlled by said first contact to interrupt the current flow through the electrical heating element; second relay means controlled by subsequent actuation of said second contact for controlling said first relay means along a path independent of said first contact to maintain interrupted the current fiow through the electrical heating element, and a time-lag relay, said second relay means being controlled by said second contact through said time-lag relay to control said first relay means only after a selected time interval.

5. A control system for controlling the current through an electrical heating element including in combination: temperature sensitive indicating means having a highlimit contact adapted to be closed when the indicated temperature of the heating element exceeds a first selected value and having a low-limit contact adapted to be closed when the indicated temperature of the heating element drops below a second selected value; circuit means for producing a current flow through the electrical heating element; first contact means included in said circuit means to discontinue the current flow through the heating element upon the opening thereof; first relay means controlled by said high-limit contact to open said first contact means when said high-limit contact is closed; circuit means including second contact means for establishing an energizing circuit to said first relay means independent of said high-limit contact; and second relay means controlled by said low-limit contact to actuate said second contact means and establish said independent energizing circuit to said first relay means upon the closure of said low-limit contact.

6. A control system for controlling the current through an electrical heating element including in combination: temperature sensitive means having a high-limit contact adapted to be actuated when the indicated temperature of the heating element exceeds a first selected value and having a low-limit contact adapted to be actuated when the indicated temperature of the heating element drops below a second selected value; first circuit means for producing a current flow through the electrical heating element; means controlled by said high-limit contact for controlling the operating condition of said first circuit means upon the opening and closure of said high-limit contact; second circuit means responsive to a control effect for deactivating said first circuit means; and means con- References Cited in the file of this patent trolled by the closure of said low-limit contact subse- UNITED STATES PATENTS quent to a closure of sand hlgh-hmlt contact for establishing an independent control efiect on said second cir- 1,715,750 Gano June 1929 cuit means so as to maintain said first circuit means in 5 1,810,172 Hayes June a deactivated condition despite subsequent actuation of 19911-280 HYneS 12, 1935 the 1 v 1imit ontact Mlner 16, 2,683,793 Gilmont et al. July 13, 1954 2,743,933 Baines May 1, 1956 10 2,788,416 Kilbury Apr. 9, 1957 

